1. Field of the Invention
This invention relates generally to brazing material and brazing fluxes, and more specifically to flux-coated or flux-cored brazing material.
2. Related Art
Various methods are known for joining metal components, including mechanical bonding, adhesive bonding, soldering, welding and brazing. Although brazing, soldering and welding are similar, there are important differences. Soldering is generally performed at lower temperatures (below 450 degrees Celsius), but does not produce as strong a joint. Welding is a high-temperature process in which the two metals to be joined are actually melted and fused together. Brazing is a method of joining two pieces of metal together with a third, molten filler material. Welded and brazed joints are usually at least as strong as the metals being joined. The welding process is preferable for applications which benefit from or require highly localized, pinpoint heating. Brazing is particularly useful in more difficult applications, such as joining of larger areas, linear joining, and joining metals or alloys having different melting points.
In brazing, the components to be joined are assembled so that there is a small gap, the so-called “joint gap,” between their mating surfaces. The components are heated (or at least heated in the region of the proposed joint) to a temperature above the melting point of the brazing material but below the melting point of the components to be joined (or, in the case of two or more components made of dissimilar metals or alloys, below the lower or lowest inciting point of any of the components to be joined). Heat may be provided by torch, furnace, induction or any other heating method that may used in joining components. During joining, the brazing material melts, wetting the surfaces of the components being joined, and is drawn or held in the joint gap by capillary action. Upon cooling, the brazing material solidifies, forming a metallurgical bond between the surfaces of the joined components.
Brazing may be used to join metal-to-metal, alloy-to-alloy, metal-to-alloy, metal-to-ceramic, alloy-to-ceramic, or ceramic-to-ceramic. Ceramic components may be coated with metals or alloys prior to brazing. Brazing materials frequently melt at temperatures above 425 degrees Celsius. Brazing materials may be comprised of one or more base metals, or eutectic mixtures or alloys thereof, such as aluminum, copper, gold, platinum, silver, tin, phosphorous, palladium, nickel, manganese, zinc, cadmium, chromium, boron, silicon, iron, carbon, sulphur, titanium, zirconium, tungsten, cobalt, molybdenum, niobium, selenium, lead, palladium, bismuth, beryllium, lithium and indium; other metals, metal alloys or minerals may also be used. A brazing material may be referred to as “brazing alloy,” “brazing material,” “brazing compound,” “brazing metal,” “brazing filler” or “filler metal.” Throughout this application, any and all materials, elements, compounds or compositions used as brazing materials are referred to herein as “brazing material” or “brazing materials.”
It is well known in the art that it is necessary to prepare the surfaces of the components to be joined prior to applying the brazing material, so that the brazing material adheres to the surfaces to be joined. When components or surfaces are joined by brazing, it is preferable that both the brazing material and the joint area of the component surfaces are free from oxide films that may degrade the strength of the brazed joint. This may be done by carrying out the brazing operation in a reducing atmosphere, such as in a furnace. However, when brazing is done in air a flux composition or a flux compound (referred to as a “flux,” “brazing flux” or “flux cover”) is used to eliminate existing oxides or inhibit oxide films from forming on the brazing material and the surfaces of the components being joined. Thus, the flux must be capable of removing metal oxides at pre-selected brazing temperatures while remaining substantially inert with respect to the brazing material. Since fluxes are usually reactive (e.g., capable of removing oxides), the flux should be transformed to its molten state at or near the melting temperature of the brazing material. The flux is first applied to the surfaces of the components to be joined and is then activated to remove oxides and clean the surfaces by the application of heat at or around the joint.
Although the principal purpose of the flux is to eliminate or inhibit the oxidation (formation of oxides) of the brazing material and of at least selected areas of the surfaces of the components being joined, the flux also must melt and flow at a temperature below the melting point of the brazing material, wet the surfaces of the components and brazing material, facilitate the wetting of the components by the molten brazing material, and be capable of being displaced by molten brazing material.
Fluxes generally comprise a eutectic mixture of borates (including, without limitation, fluoroborates), fluorides (including, without limitation, bifluorides), chlorides, or salts thereof and one or more of the alkali metals, and are typically highly corrosive and hygroscopic in nature so that the flux adequately cleans the surfaces to be joined. Nonhygroscopic and noncorrosive flux compositions are known in the art. Known fluxes include those described in U.S. Pat. Nos. 6,395,223, 6,277,210, 5,781,846 and 4,301,211. The entire content of each of these four patents is hereby incorporated by reference. Fluxes in the form of a liquid, solid, powder, slurry or paste may be applied to a brazing material or components to be joined.
Various methods are used to apply flux to the joint area and to the external surfaces of the components to be joined. Usually, the flux is applied to the surfaces to be brazed and the surfaces are heated to allow the flux to melt, flow and coat the surfaces. It is well known for the brazing flux in the form of a powder or paste to be applied to the joint area when the components are cold. The joint area is then heated until the brazing temperature is reached, and then the brazing material is applied. Various methods are used to apply brazing material to a joint, including, without limitation, insertion of the brazing material (in the form of a rod, wire, strip, disk, sheet, sheath or other form factor) into the entirety or a portion of the joint gap, upon which heat from the adjacent components begins to heat and thereby melt the brazing material. Alternatively, brazing material may be positioned at the mouth of the joint gap by melting the end portion of the brazing material.
Linear brazing materials in the form of a “brazing rods” or “brazing wires” are well known in the art and include non-circular linear forms such as sheets or strips. A brazing rod is a fixed length brazing material generally of approximately 20 inches or less. Linear brazing materials may be formed into circular or quasi-circular shape (e.g., oval, elliptical, hexagonal, semi-circular or “U”), loose coils, flat shapes (e.g., disks), conicals, saddles, bowls or other custom shapes. A brazing wire is a brazing material of continuous length. For purposes of this application, “continuous length” means a length greater than approximately twenty inches. Neither a bare brazing rod nor a bare brazing wire contain a flux core or flux coating.
Due to the corrosive, hygroscopic nature of many fluxes and the residual or excess flux that results from various methods used to apply the flux, in many applications it is necessary or desirable to remove any residual flux or flux residue from the joined parts in order to prevent or limit corrosion of the joined components. The removal of residual flux increases the overall product cost due to the additional cleaning steps and the cost to dispose waste resulting from the cleaning process.
Flux-coated brazing rods and flux-cored wires have been developed to eliminate the separate steps of applying the flux to the joint and removing and disposing of residual flux, thereby reducing the cost of manufacture. Flux-coated brazing materials have flux pre-applied to an exterior or exposed surface of the brazing material. Flux-cored brazing materials have flux pre-applied to on an interior surface, such as a channel, core, groove or other hollow form or cavity within a brazing material. Flux-coated and flux-cored brazing rods or wires may be made by first mixing a brazing flux composition, for example with water or an organic solvent or a liquid or semi-liquid binder to form a flux paste composition, a solid flux composition or a flux powder composition (such as by milling, crushing or pulverizing a solid flux). Binders commonly used include acrylic resins (e.g., 1-methoxy-2-propanol-acetate) and synthetic rubber compounds (e.g., butylpolybutadiene, polyisoprene, butadienestyrene and polyisobutylene). The flux paste may be applied to a brazing material using an extrusion press to extrude a concentric coating of the flux paste composition of a desired thickness onto the brazing rods and the coated rods are then baked to harden the flux coating.
Alternatively the flux paste or powder may be deposited within a core, notch, groove, hole, crevice, cavity or other hollow area within a brazing material, to form a brazing material form, e.g. a flux-cored wire or sheath of brazing material, for example as described in U.S. Pat. Nos. 5,781,846 and 6,395,223, owned by Omni Technologies Corporation. In flux-cored and flux-coated brazing materials, the surfaces of the components to be joined are heated and the flux-cored brazing material form is brought into contact with the heated surfaces, causing the flux to melt and flow and thereby causing the brazing material to melt and flow.
Continuous length flux-coated brazing materials are not currently known in the art. In many brazing applications, the brazing process is performed in a limited physical space and it is often necessary or desirable to bend, curl, angle or otherwise deform a linear brazing material (or to have a brazing material that is pre-formed in a bent or deformed shape) so that is may be appropriately positioned with respect to the joint gap and the components to be joined. A disadvantage of currently available flux-coated and flux-cored brazing rods or wires made in the above-described manner is that the fluxes are relatively brittle. Thus, when known flux-coated or flux-cored brazing materials are curled, bent or deformed (e.g., during transit, storage, handling or use) the flux coating or flux core easily cracks, fractures, peels or chips so that it becomes a non-continuous flux and portions of the flux coating or flux core may detach from the rods or wires. When the flux coating or flux core becomes detached and non-continuous, it loses its usefulness and effectiveness because it may produce a joint having less mechanical bond or strength.
A further disadvantage of known flux-coated and flux-cored brazing materials is that the brittle flux-coating or flux-core does not allow the brazing material to be coiled, spooled, wound or manufactured into rings or other form factors that may be formed from the flux cored or coated wire or rod into a substantially circular, oval or elliptical shapes. When the brazing material is formed into a circular, oval or elliptical shape it causes the brittle flux coating or flux core to crack, peel, fracture (which permit moisture to enter the flux) and possibly detach from the brazing material. Thus, the length of the rod or wire is limited to shorter lengths (generally less than 20 inches) and non-continuous forms that cannot be transported and handled in spooled, coiled, wound, rolled form or produced in other forms capable that permit the brazing material to be packaged, transported, stored or used in compact or compressed form.
Another disadvantage is that current flux coating processes only lend themselves to coating brazing material lengths less than or equal to approximately 20 inches, such as rods, due to the required post-cure step of baking the coating to cure or harden the flux—a step necessary to impart durability to the flux coating. The currently available form of flux coated brazing materials is limited to a rod of approximately 20 inches or less, thus limiting the usefulness of the length and causing waste, as the last inch of the rod is generally discarded as being too small to effectively use.
Accordingly, there is a need for a coated or cored brazing material that may be bent, curved, angled, curled, conformed or otherwise deformed in the brazing process. There is also a need for a continuously coated or cored brazing material of a continuous length that has a durable and flexible flux composition so that it may be spooled, coiled, wound, conformed, made or deformed into other circular or quasi-circular, non-linear or other form factors (e.g. rings, disks or ribbons). There also is a need for a durable, flexible flux composition that effectively prepares the surfaces to be joined, is clean burning and that may be deposited on the surface or core of a brazing material. Additionally, there is a need for a method of preparing a flux-coated or flux-cored brazing material that does not require post-cure heating or baking, so as to reduce manufacturing costs. There is also a need for a flux-coated or flux-cored brazing material that will provide for a wide selection of brazing base metal or alloy compositions and may include coated brazing materials having customized base metal properties. The present invention is directed to overcoming one or more of the problems set forth above.